Multimode interference (MMI) couplers are used for a wide variety of applications in photonic integrated circuits (PICs) such as splitters, combiners, and multiplexers. These applications take advantage of the variability of the refractive index of MMI material, which can be heated as a means of controlling its behavior.
As one of the basic MMI couplers, a 2×2 3 dB MMI coupler is a fundamental building block of photonic integrated circuits (PIC). FIG. 1A is a schematic diagram illustrating a conventional 3 dB 2×2 MMI coupler 100 having an MMI region 111 of a rectangular shape defined by a region length 112 equal to LMMI and a region width 114 equal to WMMI. The coupler 100 may be terminated by two pairs of ports at opposite ends of the MMI region 111, one input pair of a first port 101 and a second port 102 at a first end 113 of the MMI region 111, and another pair of third port 103 and fourth port 104 at a second end 115 of the MMI region 111. The operating principle of such a coupler is defined by well-known MMI self-imaging theory. When a single-mode optical signal is launched at either the first port 101 or the second port 102, the 2×2 coupler 100 behaves as a 3 dB power splitter, such as a Y-junction. Under such behavior, the single-mode optical signal propagates through the MMI region 111, resulting in two imaging mode optical signals with a 90-degree phase difference to emerge, one at each of the third port 103 and the fourth port 104. This 90-degree phase difference between the two output ports is an attractive feature in many applications such as broadband switches and coherent communications. The broadband wavelength response also makes the 2×2 MMI coupler 100 a better candidate for directional 3 dB couplers. Each pair of ports (101 and 102) or (103 and 104) at either end of the MMI region 111 is spaced apart by a port gap 117 equal to Dgap. According to MMI self-imaging theory, Dgap must at least be equal to ¼ WMMI. In conventional prior art practice, the region width WMMI and the region length LMMI are tuned when designing the 2×2 MMI coupler 100 by finding a self-imaging point that meets a specific desired performance
FIG. 1B illustrates another configuration of a prior art rectangular K×M MMI coupler 200 with a rectangular MMI region 211 for applications requiring more than two ports at either a first end 213 or a second end 215 of the MMI region 211. The coupler 200 is terminated by a first set of K ports 201 at the first end 213, and a second set of M ports 202 at the second end 215, where K and M are integers greater than 0. The K×M coupler 200 follows similar operating principles to those known for the 2×2 coupler 100 shown in FIG. 1A.
With reference to FIG. 1C, trapezoidal coupler geometry is also conventionally used for the MMI region 311. FIG. 1C illustrates a K×M MMI coupler 300 with a trapezoidal MMI region 311 terminated by a first set of K ports 301 at a first end 313 thereof, and by a second set of M ports 302 at a second end 315 thereof, where K and M are greater than 1. The port positioning in trapezoidal couplers is conventionally determined by the self-imaging theory disclosed by Soldano, Lucas B., and Erik Pennings, in a paper entitled “Optical multi-mode interference devices based on self-imaging: principles and applications” published in Lightwave Technology, Journal of 13.4, 1995, pages 615-627, which is incorporated herein by reference.
Performance of the aforementioned prior art couplers, in terms of metrics, such as insertion losses, phase error and bandwidths, can be tuned only by modifying the width value WMMI and length value LMMI, thus leaving little room for design flexibility, in view of the fixed geometry of rectangular and trapezoidal MMI regions. Furthermore, rectangular and trapezoidal MMI region geometries disadvantageously introduce excess loss when coupling light out of the MMI region into a waveguide. In addition, the geometry and symmetry properties of the MMI region are susceptible to significant deviations from the intended dimensions as a result of variations in the fabrication process from run to run or even from wafer to wafer in lithography, etching, wafer thickness, and the like, thereby undermining the power balance and increasing the phase error of an optical signal propagating through the MMI region.
Accordingly, there is a need for more flexibility in MMI coupler geometry, providing additional features that can be configured for improving performance, in terms of lower insertion loss, power imbalance, phase error, and wider broadband performance, while maintaining a small footprint suitable for large-scale photonic integration.